Add ice and some water to a large bowl. Place a slightly smaller bowl in the larger bowl with ice (to create an ice bath). Set aside.
In a small saucepan combine chocolate and water. Cook and whisk over low heat until melted and smooth.
Pour mixture into the smaller bowl nested in the bowl of ice water. Whisk rapidly for 6 to 8 minutes or just until mixture gets lighter in color and has the consistency of pudding. Immediately remove from ice bath. Watch the texture as you whip and make sure not to over-whip as it will make the mousse grainy. If the mousse becomes grainy, transfer it back into the pan, reheat until half of it is melted, pour it back to the mixing bowl ice bath and whisk again. Serve immediately.